The present invention pertains to a gearbox, typical of those in which the changes in ratios are obtained by moving a selector, or cottering rod, designed such that a hollow shaft is engaged in rotation with one of the gears of a series of gears mounted loosely on this shaft.
More precisely, the present invention pertains to a gearbox, comprising at least: one primary shaft and one secondary shaft, one of these shafts forming a first hollow shaft perforated by an axial bore; at least two pairs of gears, each of which comprises an input gear and an output gear which engage each other and which equip the primary and secondary shafts, respectively; a first cottering rod mounted mobile in the axial bore of the first hollow shaft and selectively adopting at least a first or second axial position; and a first set of balls comprising at least the first and second rows of balls; the input and output gears having between them, for two different pairs of gears, respective ratios that are different from each other, the first and second pairs of gears including the first and second idling gears, respectively, mounted rotating on the first hollow shaft, and each of which has internal splines, the first hollow shaft having the first and second rows of orifices emerging, respectively, opposite splines of the first and second idling gears, and in which are respectively engaged the balls of the first and second rows, and with the first cottering rod having a radial protuberance selectively engaging the balls of the first or second row in the splines of the first or of the second idling gear, according to which the first rod is in its first or its second axial position.
A gearbox of this type is described, e.g., in the document: Ulm, M., F. Lacher, and F. Graf: xe2x80x9cThe Electronic [sic] Gear Control 2000xe2x80x94Market and Technical Trends,xe2x80x9d VDI Reports 1170, 1994.
Despite the advantages that namely their simplicity provides, the prior-art gearboxes of this type are not reliable enough to be used on sedan motor vehicles, their application therefore being limited to two-wheel drive vehicles with relatively low power.
In this context, the object of the present invention is to propose a gearbox having a simple structure, but nevertheless having a reliability compatible with its application to four-wheel drive vehicles.
To this end, the gearbox of the present invention, otherwise in conformity with the generic definition that the above preamble gives it, is essentially characterized in that it comprises at least a first cage fixed in rotation relative to the first hollow shaft and arranged between the first hollow shaft and the first and second idling gears, in that this first cage consists of segments stacked according to one axial direction and defining between them a set of gaps comprising at least a first and a second gap opposite which are respectively arranged the balls of the first and second rows, in that the segments are mounted sliding according to the axial direction to permit each of the first and second gaps to selectively adopt a minimum width for which these first and second gaps block, respectively, the balls of the first and second rows to be engaged, respectively, in the splines of the first and second idling gears, and a maximum width for which the first and second gaps respectively enable the balls of the first and second rows to be engaged, respectively, in the splines of the first and second idling gears and in that the segments are mounted sliding with a total effective clearance less than twice the difference between the maximum and minimum widths, which results in that two different gaps never adopt their maximum width at the same time.
Thanks to such an arrangement, the gearbox of the present invention is protected against the risk that the balls might come out of their housing under the effect of the centrifugal force.
According to one possible embodiment of the present invention, the cage segments are, e.g., enclosed, according to the axial direction, between two pretensioned springs.
Preferably, the first and second pairs of gears include the first and second connected gears, respectively, engaging with the first and second idling gears, respectively, and integral in rotation with that of the first and second shafts which is not formed by the first hollow shaft.
If the primary shaft forms a first hollow shaft the secondary shaft may form a second hollow shaft and may itself carry at least two idling gears, this second hollow shaft being perforated with an axial bore in which are housed a second cottering rod, a second set of balls and a second cage which are respectively equivalent to the first cottering rod, the first set of balls and the first cage.
To economically increase the number of gears, the first hollow shaft may carry gears which are connected and are integral in rotation with this first hollow shaft.
If the gearbox comprises at least three pairs of gears and must permit the direct selection of a gear from a nonadjacent gear it may be suitable to provide that the different rows of balls of the first set of balls comprise at least a same certain number of balls, that the radial protuberance of the first cottering rod be at least traversed by axial notches in a number equal to the certain number of balls and each of which forms a housing for one ball, that the first cottering rod is connected in rotation, by the axial splines, to a phase control sleeve selectively mobile according to the axial direction, and that the phase control sleeve is itself connected in rotation to the first hollow shaft by a helical connection.
Moreover, it may thus be suitable to provide that the orifices made in the first hollow shaft at least be arranged in staggered rows.
To provide the gearbox with a reverse gear, one of the pairs of gears may comprise an input gear and an output gear, which only engage each other via a third gear, carried on an additional axis relative to the primary and secondary axes.
The output of the gearbox of the present invention may be optimized by providing that the idling gears carried by a same shaft are separated from each other by pin stops.
Finally, in a simple embodiment of the gearbox of the present invention, the segments of the first cage are, e.g., indexed in rotation with each other by means of cutouts and of complementary tongues engaged in these cutouts.